RDA does not exclude MTL usage. Specifically, very tight RDAs also exist. Although, I don’t know what is currently available?
It is true that one device is enough, but well… There is Squonk, there is Boro, there is SBS, from all of these there is mech, there is semi-mech… And I haven’t even mentioned the basic box Mods yet. But another important factor is the battery size (18650, 21700, just to mention the two most common ones). Right, after all this comes the RDA, RDTA, RTA… Huh… It’s a lot even just to list them, especially if you have them all at home… 